Chemical and biological sensors [1-5] are sensing elements providing direct information on ion or molecular composition of the medium (solution) the sensor is immersed in, without its sampling and preconditioning for analysis. Sensors are used in combination with any analytical registering device. To be used for determination of microcomponents in solutions, the sensors should possess ability for selective adsorption (sorption) of components being determined from these solutions, as well as ability to accumulate these components to concentrations exceeding the detection limits of the registering analytical device. If it is necessary to provide for routine monitoring (tracking the changes in microcomponents concentrations) of water stream being analyzed in in-line (right in the stream) or on-line (in bypass stream) modes, the sensors should possess satisfactory kinetic response—an ability for fast attainment of equilibrium accumulation in a characteristic time lesser than that of substantial concentration changes in the stream analyzed. The heart of any sensor is immobilized (attached to accessible surface of the sensor) active substance, able to interact selectively with components being determined.
Sensor varieties comprise chemical or biological chips [3-5]—analytical sensors containing set of different active substances in one sensor or set of sensors having different characteristics, each one containing active substance of a certain type. Chemical or biological chip provides information on the mixtures being analyzed not in the numerical form as a response to single measurement, but in the form of some pattern—two-dimensional or three-dimensional image being the compact and exact characteristic (“fingerprint”) of the mixture in whole. Although each point of such an image, characterizing presence and concentration of one or another substance, may not be quite correct, the chips allow to ensure essentially full selectivity and unambiguity in analysis due to multiple duplication of such points and to additional information in the form of integral image. The development of chemical and biological chips have began just recently. In literature, notions of electronic (chemical) “nose” or electronic (chemical) “tongue” are also used in respect of chemical chips, in order to underline that selectivity, similar to living organisms, is attained due to a set of sensors having different characteristics [6].
Different types of known chemical sensors [1] include: electrochemical (including potentiometric transducers, such as ion-selective electrodes); electrical sensors on basis of field transistors and other devices, magnetic sensors, thermometric sensors, as well as sensors sensitive to selective component accumulation due to changes in piezoelectrical or acoustical characteristics. Principal drawback of said analytical sensors lies in limited range of the components determined—virtually for each component separate sensor should be designed having certain type of active substance. Besides, it is difficult to create biological sensors on base of approaches specified.
Said drawbacks are eliminated in optical and X ray fluorescent sensors [2], wherein active substance may possess group selectivity to a large number of inorganic, organic or biologically active components. After sorption of these components on immobilized active substance, accessible surface with active substance is treated with exciting radiation by means of UV laser or X ray radiation source. In the first case, fluorescence (luminescence) spectrum is observed in the visible region, and in the second one—X ray fluorescence spectrum. Owing to analytical possibilities of methods specified, allowing to observe separately spectral bands of the components being determined, possibility appears for simultaneous analysis of multicomponent mixtures.
Fluorescent sensors are known in which active substance, interacting with environment components being determined, is applied on (impregnated) or chemically linked to membranes or microgranules of solid porous materials forming sorption layer comprising a large number of monolayers [7]. Principal drawback of said devices lies in the fact that they may not be used as chips allowing to obtain a separate signal from each active surface portion of the sensor.
The drawback specified is eliminated through use of monolayers of microgranules of solid active substances or microvessels with liquid active substances. Fluorescent sensors (biochips) are known [8, 9], wherein active substances, selectively interacting with biologically active macromolecules from the medium being analyzed, are placed in a certain regular manner into channels or pinholes, cut in a special carrier made of glass, quartz, ceramics, plastic or other inert material by lithography or other methods. At present, number of microareas with active substances attainable in such devices doesn't exceed several thousand units. This leads to decrease in sensitivity (detection limits) of the analysis utilizing X-ray fluorescence method.
The most close to the device proposed in technical essence is a fluorescent sensor on basis of multichannel structure, as described in [10]. This sensor is obtained by sintering of a bundle comprising a large number of optical fibers, each one consisting of two coaxial layers formed by two grades of glass or quartz or polymer. One of the end faces of the bundle obtained is treated with chemical substances for etching the internal layers (to a small depth ca. 10 micron) in each fiber, and microspheres of solid active sorbent substance or solid inert substance coated with an active reagent are placed and fixed in the channels formed (“microwells”). Monolayer of microgranules is obtained by way of one microsphere being placed to each channel. Distribution of microspheres in microchannels is achieved by using ultrasonic or other agitation from suspension in volatile liquid, which is then evaporated. Fixation of microspheres in channels is achieved through synthesis of films on surface of the multichannel structure end face from organic substances having different permeability. Another method of fixation involves distribution of microgranules in channels from liquid, in which granule doesn't swell, followed by treatment with another liquid, in which said granule swells and gets fixed. Thus, the device described is a fluorescent sensor on basis of multichannel structures having open microchannels, each one containing sorbent microgranule, in one of the end faces.
Principal drawbacks of said device comprise: complex manufacturing technology, unsatisfactory kinetic characteristics, associated with blocking of substantial part of microgranules surface during their fixation, as well as limited nature of analytical objects, associated with necessity to restrict swelling limits of microgranules in order to preserve integrity of the device. Another drawback of the device specified lies in impossibility to achieve low detection limits for X-ray fluorescent analysis with the use of such sensors. This is due to the fact of external and internal layers of optical fibers being comparable in thickness, so that number of sorbent granules per unit surface of the end face is insignificant, resulting, correspondingly, in equally insignificant density of adsorbed substance being analyzed per unit surface of the end face.